The present invention relates to an automatic frequency control (AFC) method and circuit therefor, and more particularly, to a digital type of automatic frequency control method and circuit therefor.
Generally, in an image processing system such as a video cassette recorder, an automatic frequency control (AFC) function is used to generate a frequency down-converting carrier color signal Fcu (i.e., 629 KHz in the case of NTSC systems) in accordance with a horizontal sync signal. Such an AFC function is performed by the same circuitry for both reproducing and recording operations. As the reference signal for this function, however, the recording operation uses a horizontal sync signal (Hsync) detected from an input video signal, and the reproducing operation uses an Hsync signal detected from a reproduced video signal.
The reproduced Hsync signal generates a jitter phenomenon due to the characteristics of the particular head and tape used in a given apparatus. Accordingly, the originally recorded video signal is very difficult to reproduce correctly. Therefore, the frequency down-converting carrier color signal, which is generated in accordance with the minute frequency variations of the horizontal sync signal due to the jitter phenomenon, is controlled by the above automatic frequency control function, so that an original video signal may be reproduced correctly.
FIG. 1 is a block diagram illustrating a conventional analog type of automatic frequency control circuit.
Referring to FIG. 1, a horizontal sync separator 11 detects a horizontal sync signal whose frequency is f.sub.H from a video signal (Vi) input through an input node 10, and applies the detected signal to a phase comparator 12. Phase comparator 12 compares the phase of the output from horizontal sync separator 11 with the phase of a horizontal sync signal (whose frequency is also f.sub.H) output from a second frequency divider 15 and converts the difference into a DC voltage. This difference is applied to a voltage-controlled oscillator (VCO) 13 which produces an output signal whose basic frequency is 160f.sub.H, and varies the frequency of the output signal in accordance with the received DC voltage. A first frequency divider 14 frequency-divides the output (160f.sub.H) signal of VCO 13 by four and applies the resultant 40f.sub.H (i.e., the down-converted color signal carrier) to an output node 16 and second frequency divider 15. Second frequency divider 15 frequency-divides this 40f.sub.H signal by 40, so as to provide a feedback signal (f.sub.H) to phase comparator 12.
The above conventional automatic frequency control circuit can be used when an analog type of video signal is processed, but cannot be used when a digital type of video signal is to be processed.